Fix hwdevice being destroyed before context causing sigsegv on AMD

[HomerSp]

Description

This fixes a bad sigsegv on AMD GPUs (maybe others too, though I've only seen it reported on AMD) running on Linux. Basically, because of the order of the members in session_t, hwdevice would be destroyed before the avcodec context. This causes problems because the context relies on the hwdevice still being active - as in, when the hwdevice is destroyed the dri library is unloaded, and va calls methods in there.

Here's a backtrace of the sigsegv:

#0  0x00007fffe6f39630 in  () -> (this is supposed to be si_resource_destroy in radeonsi_drv_video.so but because it's unloaded it no longer exists)
#1  0x00007fff7835dea1 in pipe_resource_destroy (res=<optimised out>)
#2  pipe_resource_reference (src=0x0, dst=0x5555578c0e88)
#3  vlVaDestroyBuffer (ctx=0x555556997b30, buf_id=5)
#4  0x00007ffff2224831 in vaDestroyBuffer (dpy=0x5555569979c0, buffer_id=buffer_id@entry=5)
#5  0x00007ffff6e78013 in vaapi_encode_free_output_buffer (opaque=0x5555563c3f00, data=0x5 <error: Cannot access memory
#6  0x00007ffff6631a5f in buffer_pool_free (pool=<optimised out>)
#7  av_buffer_pool_uninit (ppool=ppool@entry=0x555556a58c88)
#8  0x00007ffff680f656 in ff_vaapi_encode_close (avctx=0x5555563c3f00)
#9  0x00007ffff680dca2 in avcodec_close (avctx=avctx@entry=0x5555563c3f00)
#10 0x00007ffff6d2890d in avcodec_free_context (pavctx=0x7fffffffd288)
#11 0x0000555555dc5a97 in video::free_ctx(AVCodecContext*) (ctx=0x5555563c3f00)
#12 0x0000555555de1f3e in util::Destroy<AVCodecContext*, void, &video::free_ctx>::operator()(AVCodecContext*) (this=0x7fffffffd468, p=0x5555563c3f00)
#13 0x0000555555ddb5a2 in util::uniq_ptr<AVCodecContext, util::Destroy<AVCodecContext*, void, &video::free_ctx> >::reset(AVCodecContext*) (this=0x7fffffffd460, p=0x0)
   
#14 0x0000555555dd518f in util::uniq_ptr<AVCodecContext, util::Destroy<AVCodecContext*, void, &video::free_ctx> >::~uniq_ptr() (this=0x7fffffffd460, __in_chrg=<optimised out>)
   
#15 0x0000555555dd4792 in video::session_t::~session_t() (this=0x7fffffffd460, __in_chrg=<optimised out>)
#16 0x0000555555de4725 in std::_Optional_payload_base<video::session_t>::_M_destroy() (this=0x7fffffffd460)
#17 0x0000555555ddde5d in std::_Optional_payload_base<video::session_t>::_M_reset() (this=0x7fffffffd460)
#18 0x0000555555dd7600 in std::_Optional_payload<video::session_t, false, false, false>::~_Optional_payload() (this=0x7fffffffd460, __in_chrg=<optimised out>)
#19 0x0000555555dd428e in std::_Optional_base<video::session_t, false, false>::~_Optional_base() (this=0x7fffffffd460, __in_chrg=<optimised out>)
#20 0x0000555555dd42ae in std::optional<video::session_t>::~optional() (this=0x7fffffffd460, __in_chrg=<optimised out>)
#21 0x0000555555dcbecc in video::validate_config(std::shared_ptr<platf::display_t>&, video::encoder_t const&, video::config_t const&) (disp=
    std::shared_ptr<class platf::display_t> (use count 1, weak count 0) = {...}, encoder=..., config=...)
#22 0x0000555555dcc42b in video::validate_encoder(video::encoder_t&) (encoder=...)
#23 0x0000555555dcddab in video::init() ()
#24 0x0000555555c5c327 in main(int, char**) (argc=2, argv=0x7fffffffdac8)

Screenshot

Include screenshots if the changes are UI-related.

Issues Fixed or Closed

• Fixes Segmentation fault with AMD GPU on start #79

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the documentation blocks for new or existing components

[HomerSp]

I've updated it slightly so the order of the members won't matter, instead the session_t destructor will ensure they're destroyed in the correct order.

[TheElixZammuto]

I can't test this, but both code and rationale seems reasonable. LGTM